This application is based on patent application Ser. No. 2000-353491 filed Nov. 20, 2000 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an image forming apparatus provided with a plurality of full line heads, relative positions of which are fixed, upon a liquid droplet being ejected onto a print medium, the print medium moving by adsorption conveyance such as suction or electrostatic adsorption as medium conveyance, and more particularly relates to solving problems associated with a cockling produced on the print medium to which the liquid is applied.
2. Description of the Prior Art
A cockling occurring on a sheet of paper that is a kind of a printing medium on which liquid droplets are applied, i.e., deformation due to water imbibition by fibers is conventionally recognized as a difficulty against liquid ejection and printed image formation. The difficulty has been recognized in a serial scan type printer (hereinafter, simply referred to as a serial system) using a carriage that moves with respect to the suspended medium while placing thereon a liquid ejecting head. Japanese Patent Application Laid-open No. 6-115068 discloses a method of solving the difficulty with the prior art wherein a rib for permitting a cockling to occur on the medium is formed on a plate shaped structure for holding in contact a medium back surface called a platen.
In a system (hereinafter referred to a full-line system) where upon ejecting liquid droplets a full-line type head is employed with a ejection opening disposed over the entire width of a print medium to convey the print medium, floating of the print medium itself upon conveyance is a problem rather than the amount of displacement of the print medium caused by the foregoing cockling. In the full-line system, many inventions on how securely adsorption and conveyance are performed have been applied. For example, Japanese Patent Application Laid-open Nos. 7-53081, 7-133035, and 9-254460 disclose a fact that a print medium is adsorbed to a platen with the aid of suction or static electricity.
A main technique of the prior art concerns such a platen structure that cockling produced even in the serial system may be allowed in a print region. It is the present situation in the full-line system that even a problem of the cockling itself has not been recognized much.
The present invention is to pursue optimum print conditions by clarifying correlations among respective components of an image forming apparatus capable of realizing high image quality with the aid of ejecting of liquid droplets by achieving clarification of a cockling itself not known conventionally. The present invention is further to complete a novel system capable of achieving optimum printing for a cockling.
More specifically, it is an object of the present invention to provide a novel optimum design method capable of determining requirements of the whole of an image forming apparatus by investigating a relationship among respective components from the occurrence of cockling in the aforementioned full-line system.
It is another object of the present invention to provide an image forming apparatus capable of achieving high image quality without being affected by cockling while utilizing an optimum printing gap by setting a relationship among the respective components without being affected by characteristics (surface tension, etc.) of horizontally ejecting a liquid such as an ink and characteristics of a print medium itself such as paper, which are both dominating stages of occurrence and growth of the cockling in the full-line system.
It is a further object of the present invention to provide an optimum image forming apparatus with all of the variations of the foregoing respective elements.
The present inventors have investigated variously a change in cockling height with respect to the time elapsed since application of liquid droplets by pursuing an occurrence mechanism of the cockling in an adsorptive conveyance system with respect to a change in the cockling produced owing to a difference among kinds of ink and a difference among its invasion characteristics or a difference among its infiltrate characteristics, and further differences among fibers of the print medium used and sizing agents. It has been hereby found that if the adsorptive conveyance system is employed for a time shorter than 1 second after application of liquid droplets, any cockling is substantially not produced on any print medium or with any ink. It is considered that the reason is that the mechanism of the occurrence of the cockling is due to infiltration of water constituting the ink. It has been also found that beyond one second after the application of liquid droplets, the height of the cockling is changed in response to the characteristics of each ink and the print medium. FIG. 2 illustrates this change, in which cockling heights HC (mm) are on the y axis and the time TD elapsed is on the x axis, which time elapsed takes the time of application of liquid droplets as the origin, i.e., xe2x80x9c0xe2x80x9d. Coordinates Y(1, xcex3) indicate a reference point with xcex3=0.05 in water ink. Such use of xcex3 is convenient to clarify applicability of the present invention, because xcex3 changes only slightly, even when other ink systems are employed or even though ink characteristics developed hereafter, if any, change in particular. In any case, a curved line indicating a change in cockling heights HC after the elapsed time TD=1 (sec) is as illustrated in FIG. 2.
In accordance with one view of the present invention, a tangential line HC=xcex1TD+xcex2 with respect to the curved line passing through the reference point Y(1, xcex3) passes through (1, xcex3), from which curved line xcex1, xcex2, and xcex3 can be in turn determined. A choice may be made as a matter of course by a maker of a print medium that provides such a curved line upon design of the print medium by the maker. It is herein preferable to take as an object a print medium that ensures a maximum cockling. If ink used is already determined, then the reference point Y(1, xcex3) in the individual image forming apparatus is defined by itself.
For the adsorptive conveyance means there may be typically used one in electrostatic adsorption conveyance and suction conveyance described later, in which a print medium is brought into close contact with a conveyor such as a moving belt, the accuracy of which means is practically uniform.
However, even if use is made of a belt with reduced accuracy and an adsorption system with reduced cling properties in order to reduce the manufacturing cost, the present invention can complete printing in a region where the amount of occurrence of the cockling is more reduced than the prior art. There is therefore ensured an image with higher quality than the prior art image forming apparatus without disturbance of the image by xe2x80x9crubbingxe2x80x9d.
The present invention discloses items where technical idea of the present invention is developed while taking such manufacturing and practical error fractions into consideration.
Referring here to FIG. 3, a conceptual view of a portion of the image forming apparatus illustrated in FIG. 1 is provided, which defines a head existing region (its length L). The head existing region in the present invention is defined from a ejection opening A of a head section 13B where printing is first performed up to an end B of a head section 13Y (an end which is capable of making contact with the print medium) where printing is finally performed.
A practical print system according to the present invention is adapted such that an image forming apparatus comprises:
medium-conveying means for attracting and conveying a print medium having a characteristic with which cockling is produced by application of liquid droplets;
attracting means for attracting the print medium to the medium-conveying means; and
a first liquid ejecting head for first providing liquid droplets on the upstream side in the direction of conveyance of the print medium, and a second liquid ejecting head for finally providing liquid droplets on the downstream side, the first and second ejecting heads being separated from the print medium with a gap,
wherein for a slope xcex1 and an intercept xcex2 obtained from a tangential line to a curved line which the tangential line passes through an x-y reference point (1, 0.05) obtained from the curved line in an x-y coordinate system, the curved line representing the height (y) mm of the cockling in response to the time (x) elapsed since the application of liquid droplets onto the print medium, a minimum gap H (mm) in the gap, the length L (mm) of a ejecting head existence region extending from an ejecting portion of the first liquid ejecting head up to an end of the second liquid ejecting head, and a conveyance speed V (mm/sec) of the print medium satisfy an equality:
H greater than xcex1(L/V)+xcex2, with Hxe2x89xa70.05 and xcex1+xcex2=0.05.
This is proper conditions obtained in water ink used at present.
It should be noticed that an x-y reference point (1, 0.05) of the practical print system is indicated as (1, xcex3), whereby the foregoing conditions are capable of employing xcex3 as a value corresponding to xe2x80x9c0.05xe2x80x9d, which is thus represented as a technical idea capable of also dealing with a change in ink characteristics which will be considered hereafter.
As understood from the curved line shown in FIG. 2, the time xe2x80x9c1 sec. or lessxe2x80x9d after the ejection and application of liquid droplets is optimum conditions for high image quality, so that it is more preferable to satisfy an inequality Vxe2x89xa7L.
Taking variations and error of each parameter into consideration, the present invention is collected as the following mode. An ejecting speed of liquid droplets is 8 to 10 meters or more per second, so that variations of an impact position on calculation can be ignored.
Another aspect of the present invention is in an image forming apparatus comprising:
medium-conveying means having a conveyance surface for moving at a speed V along a conveyance reference surface to convey a print medium; and
at least two liquid ejecting heads disposed at a distance in the movement direction of the conveyance surface and respectively having an ejection opening surface through which a plurality of ejection openings are opened for ejecting a liquid, the image forming apparatus forming an image by ejecting the liquid onto the surface of the print medium from the ejection opening of the liquid ejecting head disposed oppositely to the conveyance surface,
wherein an interval between the ejection opening surface of an arbitrary second liquid ejecting head located downstream first liquid ejecting head located upstream in the movement direction of the conveyance surface for firstly ejecting the liquid to the print medium and the conveyance surface
is smaller than the total sum of the maximum value of variations of the height of the conveyance surface with respect to the conveyance reference surface, the maximum error amount of the thickness of the print medium and the maximum amount of relief from the conveyance surface, and the maximum height HCX of cockling produced on the print medium owing to the liquid ejected onto the surface of the print medium, and
is set to be larger than the total sum of the maximum value of variations of the height of the conveyance surface with respect to the conveyance reference surface, the maximum error amount of the thickness of the print medium and the maximum amount of relief from the conveyance surface, and the height HC1 of the cockling produced on the print medium after the elapse of predetermined time TC1 since first ejection of the liquid to the print medium from the first liquid ejecting head, and
wherein a distance L between the first liquid ejecting head and the second liquid ejecting head is set shorter than (V/VCX)HCX when the maximum value of the growth speed of the cockling is represented by VCX, and further
the predetermined time TC1 being L/V.
Further another aspect of the present invention is in an image forming apparatus comprising:
medium-conveyance means having a conveyance surface that moves at a speed V along a conveyance reference surface to convey a print medium; and
at least two liquid ejecting heads each having ejection opening surfaces through which a plurality of ejection openings are disposed, separated away in the movement direction of the conveyance surface and are opened to eject a liquid, whereby the liquid is ejected to the surface of the print medium from the ejection openings of the liquid ejecting head facing the conveyance surface to form an image,
wherein a gap between the ejection opening surface of an arbitrary second liquid ejecting head located more downstream than a first liquid ejecting head located on an upstream side in the movement direction of the conveyance surface for firstly ejecting the liquid to the print medium and the conveyance surface
is set smaller than the total sum of the maximum value of variations of the height of the conveyance surface with respect to the conveyance reference surface, the maximum error amount of the thickness of the print medium and the maximum amount of relief from the conveyance surface, and the cockling height HC2 when the growth speed is maximum of the cockling after the liquid is firstly ejected to the print medium from the first liquid ejecting head, and
is set larger than the total sum of the maximum of variations of the height of the conveyance surface with respect to the conveyance reference surface, the maximum error amount of the thickness of the print medium and the maximum amount of relief from the conveyance surface, and the height HC1 of the cockling produced on the print medium after the elapse of a predetermined time TC1 since the liquid is firstly ejected to the print medium from the first liquid ejecting head, and
wherein a distance L between the first liquid ejecting head and the second liquid ejecting head is set to be shorter than (V/VCX) when the maximum value of the growth speed of the cockling is represented by VCX, and further
the predetermined time TC1 being L/V.
Still another aspect of the present invention is in an image forming apparatus comprising:
medium-conveyance means having a conveyance surface moved at a speed V along a conveyance reference surface to convey a print medium; and
at least two liquid ejecting heads each having a ejection opening surfaces which are disposed, separated away from each other in the movement direction of the conveyance surface and through which a plurality of ejection openings are opened each for ejecting a liquid, whereby the liquid is ejected from the ejection openings of the liquid ejecting head facing the conveyance surface onto the surface of the print medium to form an image,
wherein a gap between the conveyance surface and the ejection opening surface of an arbitrary second liquid ejecting head located more downstream than a first liquid ejecting head located on the upstream side in the movement direction of the conveyance surface for firstly ejecting the liquid onto the print medium
is set smaller than the total sum of the maximum value of variations of the height of the conveyance surface with respect to the conveyance reference surface, the maximum error amount of the thickness of the print medium and the maximum amount of relief from the conveyance surface, and a cockling height HC3 at a time TC3 which is calculated on the basis of the maximum value VCX of the growth speed of the cockling produced on the print medium since the liquid is first ejected onto the print medium through the first liquid ejecting head and when a difference between the cockling height HC3 and the actual cockling height is maximum, and
is set larger than the total sum of the maximum value of variations of the height of the conveyance surface with respect to the conveyance reference surface, the maximum error amount of the thickness of the print medium and the maximum amount of relief from the conveyance surface, and a cockling height HC1 produced on the print medium after the passage of predetermined time TC1 since the liquid is firstly ejected onto the print medium by the first liquid ejecting head, and
wherein a distance L between the first liquid ejecting head and the second liquid ejecting head is set shorter than Vxc2x7TC3, and
the predetermined time TC1 being L/V.
In this situation, the cockling height HC3 at the time TC3 estimated on the basis of the maximum growth speed VCX of the cockling is about 0.3 mm, and the time TC3 is about 1.1 sec.
Still further aspect of the present invention is in an image forming apparatus comprising:
medium-conveyance means having a conveyance surface; and
at least two liquid ejecting heads each having ejection opening surfaces which are disposed, separated away from each other in the movement direction of the conveyance surface and through which a plurality of ejection openings are opened for ejecting a liquid, whereby the liquid is ejected onto the surface of the print medium from the ejection openings of the liquid ejecting heads facing the conveyance surface to form an image,
wherein an interval between the conveyance surface and the ejection opening surface of an arbitrary second liquid ejecting head located downstream of a first liquid ejecting head which is located on the upstream side in the movement direction of the conveyance surface
is set smaller than the total of the maximum of variations of the height of the conveyance surface with respect to the conveyance reference surface, the maximum error amount of the thickness of the print medium and the maximum amount of relief from the conveyance surface, and the maximum height HCX of the cockling produced on the print medium owing to the liquid ejected on the print medium, and
is set larger than the total sum of the maximum of variations of the height of the conveyance surface with respect to the conveyance reference surface, the maximum error amount of the thickness of the print medium and the maximum amount of relief from the conveyance surface, and the height HC4 of the cockling after the passage of predetermined time TC, which is calculated on the basis of the maximum value VCX of the growth speed of the cockling and since the liquid is firstly ejected to the print medium by the first liquid ejecting head, and
wherein a distance between the first liquid ejecting head and the second liquid ejecting head is set shorter than (V/VCX)HCX, and further
the predetermined time TC1 being L/V.
In the image forming apparatus according to the present invention, the liquid may be aqueous, and the print medium may be a non-coated sheet of paper. The plurality of the ejection openings may be arranged over the entire width of a print region of the print medium that intersects the movement direction of the conveyance surface.
The distance between the first liquid ejecting head and the second liquid ejecting head located most downstream may be less than 75 mm, the movement speed V of the conveyance surface may be more than 300 mm/s, the gap between the ejection opening surface of the second liquid ejecting head and the surface of the print medium may be less than 0.4 mm.
The medium-conveyance means may include a conveyance belt. Alternatively, the medium-conveyance means may include a revolving drum, an external peripheral surface of which serves as the conveyance surface.
In accordance with the present invention, the height of the cockling grown on the print medium is predicted in response to the time elapsed after the liquid is firstly ejected on the print medium, on the basis of which a distance between an ejection opening surface of the liquid ejecting head and a conveyance surface of then same is set, and an interval between the liquid ejecting heads arranged in the conveyance direction of the print medium is set in response to the movement speed of the conveyance surface, so that the cockling produced on the print medium is prevented from making contact with the ejection opening surface of the liquid ejecting head, and further the distance between the ejection opening surface of the liquid ejecting head and the conveyance surface can be set to a necessary minimum, whereby image quality can be more improved than in a prior art multi-line printer.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.